Hard turning is a machining process applied to metallic materials with a hardness greater than 45 HRC (which corresponds to about 450 HV1), and is typically performed after a workpiece has been heat treated. In hard turning a cutting tool describes a toolpath while a workpiece rotates. The tool's axes of movement may be a straight line, or they may be along some set of curves or angles. Usually the term “turning” is reserved for the generation of external surfaces by this cutting action, whereas this same essential cutting action when applied to internal surfaces (such as holes) is called “boring”. Thus the phrase “turning and boring” categorizes the larger family of essentially similar processes. When turning, a piece of relatively rigid material (such as metal) is rotated and a cutting tool is traversed along 1, 2, or 3 axes of motion to produce e.g. precise diameters and tolerances.
A significant limitation of the widespread use of hard machining of metallic materials is the so-called “white layer” effect, which is a microscopic alteration of the as-machined surface of a workpiece which appears white under a Light Optical Microscope (LOM), which effect is produced in response to an extremely high thermo-mechanical load exerted at the as-machined surface of a workpiece by the cutting tool. Such white layers have a high hardness and are brittle compared to the bulk material of the workpiece. A darker region, or “dark layer” is also formed beneath the brittle and hard white layer by the action of thermo-mechanical loads on the workpiece. The dark layer is softer than both the white layer and the unaffected material. When high external loads are applied on such a triple-layered structure (i.e. a hard or very hard white layer, a soft dark layer and hard unaffected material) cracks may develop in the white layer between the white layer and the dark layer, or between the dark layer and unaffected material. When these cracks extend and connect together, flaking can occur.
A thermo-mechanically-affected workpiece surface comprising an etching-resistant white layer has conventionally been undesired because of high tensile and surface stresses associated therewith, such as reduced fatigue-resistance, lower fracture toughness, and/or reduced wear resistance of parts produced.
The location of such thermo-mechanically-affected layers on an as-machined surface of a workpiece is illustrated in FIG. 1. The micrograph shown in FIG. 1 namely shows a chemically etched, polished cross-sectional view of the typical subsurface microstructure of an as-machined workpiece observed under a Light Optical microscope (LOM) using a magnification of about 1000 times. The microstructure shows an outer surface or “white layer” (10) that was in direct contact with the cutting tool during hard turning. In addition, the microstructure shows a “dark layer” (12) beneath the white layer (10). The dark layer (12) is an over-tempered zone which has been exposed to a high temperature during the hard turning. Under the dark layer (12) is unaffected material which is the parent material that is unaffected by the machining process.
A white layer (10) as illustrated in FIG. 1 is formed during machining processes such as hard turning have negative effects on surface finish and fatigue strength of products. The white layer (10) is generally a hard phase and leads to the surface becoming brittle causing crack permeation and product failure. This is a major concern with respect to service performance especially in the aerospace and automotive industries. Due to the undesired properties of the white layer (10) as shown in FIG. 1, methods of removing, reducing or eliminating the white later (10) and the dark layer (12) are known in the prior art.
For example, U.S. patent application no. US 2003/0145694 discloses an apparatus and a method for reducing a thickness of a thermo-mechanically-affected layer on an as-machined surface of a hard metal workpiece being machined by a hard cutting tool exerting a thermo-mechanical load on a surface of the workpiece. The method involves reducing the thermomechanical load on the surface of the workpiece, and the apparatus includes a means for reducing the thermo-mechanical load on the surface of the workpiece.
U.S. patent application no. US 2013/0016938 concerns a rolling bearing of which the lifespan is increased by reducing brittle flaking and impression-induced flaking on the raceways of inner and outer races and the rolling elements. Steel containing 1.80-1.89% by weight of chrome (brittle flaking-resistant steel) is subjected to carbonitriding and then to hardening and tempering. The chrome reduces generation of white layers which are aggregates of carbon, thus reducing brittle flaking on e.g. the raceways due to the white layers. A residual austenite region that forms when the steel is hardened and tempered increases toughness of the steel surface, thus reducing impression-induced flaking due to foreign matter such as wear dust. By reducing both brittle flaking and impression-induced flaking, it is possible to extend the lifespan of the bearing, and reduce maintenance cost such as the cost for changing lubricating oil.